The present invention concerns liquid crystal displays and, more particularly, a novel transflective liquid crystal display providing bright viewable areas, in the activated condition, responsive either to reflected or transmitted light.
It is well-known to utilize a liquid crystal display to provide a low-power-consumption display which will not wash out when viewed in direct daylight. Displays of high brightness and acceptable contrast ratio may be provided by utilizing aa cholesteric host liquid crystal material with guest dichroic dye, as described by White and Taylor, in the Journal of Applied Physics, Vol. 45, page 4718 (1974). This form of display has acceptable viewability at moderate-to-high ambient light levels; in a low ambient light level situation, an external light source is typically required for the display to be viewable. Two external illumination methods are known: wedge illumination and back illumination. In the first technique, a light source is positioned in front, and at one edge, of the display cell, with light rays being directed by a wedge-shaped plate of glass or plastic, positioned in front of the display, toward the display cell. Light rays generally fall upon the display at a grazing angle, of approximately 15 to 20 degrees. At this incidence angle, a component of light is absorbed by molecules in the activated state, thereby degrading contrast. Further, the wedge illumination technique is relatively costly and cumbersome. Back illumination has hitherto been even less successful as, by reflection, light passes through the display twice and only once by transmission, if the display is designed to have sufficient brightness. Hitherto, no practical scheme has been available to view a guest-host liquid crystal display cell, such as a cholesteric display cell, by either transmission or reflection with sufficient contrast ratio and brightness in both the transmissive and reflective modes. Accordingly, it is highly desirable to provide a display which is transflective, i.e. viewable by both transmission and reflection with sufficient contrast ratio and brightness to provide acceptable viewing characteristics.